Atomic Layer Deposited Platinum Electrocatalysts on Chemically Functionalized Graphene

Electrocatalysts play important roles in the performance of electrochemical energy conversion devices such as fuel cells and photoelectrochemical cells. Graphene is a promising candidate for the backbone structure for electrocatalysts due to its large surface area (theoretical value of 2630 m²/g) and good electrical conductivity. Catalyst particles, e.g., Pt nanoparticles, can be integrated with graphene via wet-process route, physical vapor deposition, or chemical vapor deposition. Catalyst-graphene composite materials  synthesized via these routes, however, often suffer from low chemical/mechanical stabilities, low particle density, or non-uniform coverage [1,2].

Atomic layer deposition (ALD), which is a modified chemical vapor deposition (CVD) method, is capable of decorating dense nanoparticles chemically bonded to 3-dimensional substrates in a uniform manner. In ALD, the substrate is exposed to the alternate pulses of precursor molecules, and coated with nanoparticles or conformal thin film depending on the cycle numbers. Nevertheless, the integration of graphene with ALD catalyst particles, e.g. Pt, has been challenging because the basal plane of the pristine graphene is chemically inert due to strongly sp²-bonded carbon atoms where the nucleation cannot readily occur [3].

To this end, we present a chemical functionalization method of graphenes for the subsequent ALD platinum nucleation. We show that the low-power hydrogen plasma treatment hydrogenates the basal plane of graphene and significantly increases the chemical reactivity of the basal plane, facilitating the nucleation of the Pt nanoparticles by ALD. We systematically study the dependence of the particle density and the areal coverage on the graphene thickness and on the hydrogenation duration. While the areal coverage of Pt nanoparticles on the basal plane of the pristine graphene is negligible after 100 cycles of ALD Pt (<5 %), that of the few layer graphene hydrogenated for 2 mins hugely increases to > 80 %.

We also investigate the individual particle shape, facets, and the interface between the particle and the graphene in atomic scale using probe-corrected scanning TEM (TEAM1). We show that the ALD Pt particle has epitaxially grown on the graphene with a large density of high-index facets, which makes it a promising candidate for mechanically and chemically stable electrocatalyst with high catalytic activity.

Acknowledgment 

 We thank Precourt Institute of Energy at Stanford university for financial support.

References

[1] E. J. Yoo, et al., Nano Lett. 9(6), pp. 2255-2259 (2009)

[2] B. Seger and P. V. Kamat, J. Phys. Chem. C 113(19), pp. 7990-7995 (2009)

[3] S. Sun, et al., Sci. Rep. 3, p. 1775 (2013)